1. Field of Invention
The invention relates to a method for the integral molding of a thermoplastics synthetic part on the rim of a thin-walled body consisting of paper, cardboard or the like, particularly the injection molding of a plastics top on the edge of a tube consisting of synthetic plastics coated paper, the plasticised synthetic material being injected into the mold chamber formed by inner and outer mold parts, the mold parts being pressed against one another with force.
The invention also relates to an apparatus for integral molding, there being formed between an inner mold part and an outer mold part a mold cavity which can be brought into engagement with the nozzle of an injection moulding unit with a plasticising cylinder and an extruder worm adapted for movement therein, means being provided for pressing together the mold parts.
2. Description of the Related Art
Such an integral molding process and a similar integral molding apparatus are known in the case of machines in which liquids packages are produced, preferably even filled, sealed and then carried away in a line. In the case of such package producing machines, a tube is produced from synthetic plastics coated paper, pushed onto the mandrel on a mandrel wheel and a synthetic plastics top is integrally molding on the open end of the mandrel. The injection molding arrangement is so constructed that the mandrel constitutes the inner mold part around which, and at a distance, engage two outer mold halves which thus form the outer mold part. By virtue of the gap between the inner mold part and the outer mold part, a mold cavity is created into which synthetic plastics material is injected from the nozzle of the injection molding unit.
Where the prior art injection molding equipment is concerned, granulate is fused on an extruder worm adapted for movement in a heated plasticising cylinder and the molten material is brought into the filling space in front of the tip of the extruder worm, from whence it is injected in a measured quantity into the mold cavity described. This injection of molten synthetic plastics material into the mold cavity takes place at very high forces by means of hydraulically driven cylinder-piston units.
Synthetic plastics tops on liquids packages are kept thin-walled and the access passages for the liquid synthetic plastics material have a restricted cross-section to produce a high flow resistance which brings about the aforesaid high molding pressures. With special machines, an injection molding pressure of 1000 bars is feasible. This high pressure is propagated from the injection molding passage into all parts of the mold cavity. By virtue of the property of the plasticity in the synthetic plastics material, this high pressure has a disadvantageous effect which extends into end parts of the mold cavity. Therefore, the inner and outer mold parts must be held together with considerable force. Closure forces of 1000 tonnes are nothing out of the ordinary with relatively large machines. Such high closure forces are applied in order to keep the mold cavity substantially constant.
More accurate measurements in the mold cavity of prior art machines have however shown that the pressure which acts outwardly form the synthetic plastics material in the mold cavity actually enlarges the mold cavity during operation, producing a certain deformation. Both the mountings of the inner mold part, e.g. of the mandrel in a mandrel wheel, and also the mountings of the outer mold halves which are pivotable about bearing shafts, are exposed to high forces and are displaced minimally in such directions that the mold cavity is enlarged.
Synthetic plastics tops on liquids packages often have opening means with lines of weakness which are intended to be torn open by the end user who wishes to open the package, but which must remain reliably liquid-tight until the first opening. It has been shown that the ratio of the wall thickness at the top to the thickness of line of weakness must be very exact and that the deformation of the mold cavity due to the aforesaid high forces does produce disadvantageous dimensional shifts. Attempts have already been made by pre-set pressures to arrive at the desired final dimension of the lines of weakness but in this case during production and in operation of the packaging machine, the tolerances known to a man skilled in the art are very important. Therefore, the entire injection molding system is very critical.
For example, what is very precisely important is the quantity of synthetic plastics material injected into the mold cavity because even very tiny increases in injection quantities can lead to considerable rises in pressure. The synthetic plastics material to be plasticised must therefore be injected in a very carefully measured form which is however critical due to tolerances in the proximity switches.
Furthermore, it has been found that in prior art package producing machines minimum times are needed to bring the inner mold part into the correct position, to close the outer mold halves, to inject the synthetic plastics material, to cool it and then to open the mold halves again. The times determined by the injection molding establish the clock times attainable for the machine as a whole, so that there is no way of utilizing time saved elsewhere on the machine.
For a satisfactory configuration of a synthetic plastics top, it is desirable for the injection port to be disposed in the region of the centre of the top so that the liquid synthetic plastics material can flow radially outwardly from the centre under substantially the same conditions. As long as the liquid synthetic plastics material is flowing through the central passage, heat is supplied do that during this time the middle area of the top cannot cool. It would therefore be highly desirable to be able to reduce the flow time necessary for movement of the synthetic plastics material.